The present invention is directed to electromagnetically powered devices, and, more particularly, to powered devices such as electric toothbrushes having an electromagnetic drive unit.
One method for actuating the bristles, or other cleaning elements, of an electric toothbrush or another device having a powered handle is an electromagnetic drive positioned within the handle portion of the toothbrush or other device. The electromagnet can be actuated by a switch to alternate polarity at a desired frequency. A movable permanent magnet (or a pair of permanent magnets) is positioned proximate to the electromagnet, such that the permanent magnet is driven to oscillate at an oscillating frequency by the electromagnet when the electromagnet is actuated. A neck, which may or may not be elongated, including a workpiece such as a brush head is typically attached to the permanent magnet, such that the workpiece is driven to oscillate by the movement of the permanent magnet.
Recognizing the need to replace certain aspects of these workpieces, such as toothbrush bristles, after they are worn out or in order to provide more options, e.g., to attach a different head with a different function, manufacturers have designed replacement heads that fit onto separate electromagnetic drive units. The drive units may include the power source, switch, fixed electromagnet, movable permanent magnet and drive shaft, with the replacement heads including a neck and a workpiece such as a toothbrush head. In some instances, the replacement heads may further include the permanent magnet, drive shaft and workpiece. The replacement heads can be removably attached to the drive units, for instance, by threading or otherwise connecting a portion of the replacement head onto a portion of the drive unit.
More recently, manufacturers have attempted to control the movement of these workpieces, in order to provide a more efficient and desirable workpiece motion. For example, in the case of electric toothbrushes, manufacturers have attempted to control the movement of the cleaning elements in a rotational motion about the central longitudinal axis of the toothbrush. Difficulties arise in doing so, especially in the case of toothbrushes with electromagnetic drives, because the generally linear oscillation caused by the electromagnet must be converted into the desired rotational motion. Some replacement heads include parts that may contribute to vibrations and/or noise in the electric toothbrush that may be undesirable or annoying. In each situation, the motion of the workpiece can vary dramatically as the frequency of the electromagnet approaches the resonant frequency of the drive shaft and workpiece.